Ozone is a toxic gas produced in photochemical air pollution as a result of a complex sequence of reactions involving oxides of nitrogen, hydrocarbons and sunlight. The Clean Air Act in the U.S. and similar laws in other countries set limits on ozone concentrations in ambient air. Enforcement of compliance with the U.S. National Air Quality Standard requires continuous monitoring of ozone concentrations. Compliance monitoring is done almost exclusively by the method of UV absorbance of the Hg emission line at 254 nm. Low pressure mercury lamps provide an intense, stable and inexpensive source of radiation very near the maximum in the ozone absorption spectrum.
It is well known that ozone monitors based on UV absorbance suffer from interferences from other species that absorb at 254 nm. Volatile organic compounds (VOCs) that interfere are generally aromatic compounds. Some VOCs have a larger response at 254 nm than ozone itself. For example, Kleindienst et al. (1993) reported that the response of 2-methyl-4-nitrophenol is about 40% higher than ozone. Mercury provides a particularly strong interference because the electronic energy levels of Hg atoms are resonant with the Hg emission line of the low pressure Hg lamp used in ozone monitors. The relative response to Hg as compared to ozone depends on the temperature and pressure of the lamp and on the efficiency with which the instrument's internal ozone scrubber removes mercury, but is usually in the range 100-1000. The U.S. EPA (1999) reported that at a baseline ozone concentration of approximately 75 parts per billion (ppb), the presence of 0.04 ppb Hg (300 ng/m3 at room temperature) caused an increase in measured ozone concentration of 12.8% at low humidity (RH=20-30%) and 6.4% at high humidity (RH=70-80%) using a UV photometric ozone monitor. For dry air, Li et al. (2006) found that 1 ppb of mercury gave a response equal to approximately 875 ppb of ozone in the same model of Thermo Electron Corporation photometric ozone monitor used in the EPA study. This mercury interference can be quite large inside buildings where mercury vapor may be present as a result of past mercury spills (broken thermometers, fluorescent light fixtures, electrical switches, etc.), near mining operations and near various industrial facilities.
Another way in which mercury interferes in the measurement of ozone using ozone photometers is by adsorption and desorption from the instrument's internal ozone scrubber. These scrubbers are typically composed of manganese dioxide, charcoal, hopcalite or heated silver wool. Mercury atoms will adsorb to and accumulate on the surfaces of the scrubber material. If the temperature of the scrubber increases, or if the humidity changes, the mercury atoms may be released from the scrubber and enter the gas stream. While removal of mercury vapor from the sample stream by the scrubber will cause a positive interference, release of mercury from the scrubber will cause a negative interference. Since mercury is present at some level in all outdoor and indoor air, this interference may be responsible for much of the baseline drift that occurs in photometric ozone monitors.
A water vapor interference in the measurement of ozone by UV absorption has been described by several investigators (Meyer et al., 1991; Kleindienst et al., 1993; Leston and Ollison, 1993; Leston et al., 2005; Hudgens et al., 1994; Kleindienst et al., 1997; Maddy, 1998; Maddy, 1999; U.S. Environmental Protection Agency, 1998; Wilson and Birks, 2006). Recent studies have shown that this interference, which may amount to up to several tens of ppb of ozone, is caused by physical effects by water vapor on the transmission of light through the detection cell (Wilson and Birks, 2006). Depending on the humidity history, the solid-phase ozone scrubber can either add or remove water vapor from the flow stream during the measurement of reference lamp intensity, thereby affecting the calculated ozone concentration. Consistent with this hypothesis, it was found that reducing the mass of the ozone scrubber material greatly reduced the degree of interference (Wilson and Birks, 2006).
This invention provides a means of reducing interferences from Hg, UV-absorbing organic compounds, particles and water vapor to negligible levels by replacing the solid-phase ozone scrubber used in UV-absorbance-based ozone monitors with a gas-phase ozone scrubber. In particular, nitric oxide (NO) can be added to the sample stream to serve as the gas-phase scrubber.
The foregoing example of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.